Should I shower before I swim? Which is more detrimental to the environment – the chlorine required to clean the swimming pool if I swim in it without showering v the impact of the hot water for a pre-dip shower? It’s complicated to answer because we are comparing apples to pears (or mangoes to guavas if in India). “I shall have to investigate” I thought quoting my new hero, Frankie the Flamingo.

Swimming pools are disinfected to prevent the build-up of bacteria that could prove harmful to health, and most public swimming pools in the UK use chlorine to disinfect the water, delivered via an automatic dosing system. A decision not to shower will therefore have a direct impact (however small that may be) on how much chlorine is added.

The chlorine reacts with water to form various chemicals, most notably hypochlorous acid, which kills the bacteria and other pathogens. But it’s not quite as simple as this. Chlorine is required in its ‘free’ state, to be available to oxidise organic substances and thus disinfect the pool water. The fewer foreign substances introduced into the water, the more free chlorine residual that is available. As sweat on our bodies reacts with chlorine in the water, the chlorine changes its state to combined (or “used”) chlorine and is no longer available for disinfection. Pools are constantly being monitored to check both the levels of used and residual chlorine. If there is too little free chlorine more needs to be added. That’s quite straightforward to control. What’s more difficult is the amount of combined chlorine residual. If this becomes too high (over 0.5ppm) then the pool has to be superchlorinated (which requires a lot of chemicals and the closure of the pool).

Chlorine combined with ammonia (present in both sweat and urine) produces chloramines which cause eye irritation and an objectionable chlorine odour. So, when chlorine can be smelt around a pool it is not that there is too much chlorine in the pool, but too much combined chlorine and more chlorine must be added to increase the free chlorine level.

The question was which is better for the environment, so let’s add some figures here. Of course CO2 emissions are not the only environmental impact of producing chlorine, but they are the easiest to measure. Chlorine is produced through electrolysis of a salt-solution, a highly energy intensive process, requiring an electricity consumption of 4380 kWh/tonne chlorine (4.4kWh/kg chlorine). Assuming the chlorine was produced in the UK, where the carbon load of grid generated electricity is 0.544kgCO2/kW, every kg of chlorine contains 2.39kgCO2

A two minute hot shower requires 0.47kWh of gas. Assuming a gas condensing boiler, the shower produces 0.01 kg (10 grammes) CO2. So, one kg of chlorine in CO2 terms equates to 239 two minute showers. What I really need to know is how many grammes of chlorine are used up in reacting to the sweat on an average body. That proved a question too far, but my gut instinct is that more than 1kg of chlorine would be required to treat the sweat on 239 bodies. So, yes, I should shower before I swim.

I’m glad Lucy asked me this question. I used to think that weeing in a swimming pool was probably ok because “the water is chlorinated anyway”. Now I realise it’s a really bad idea!

February 2013 – Showers and swimming pools